The 3GPP (Third Generation Partnership Project, third generation partnership project), as an innovation trend and a target of a next generation digital communication technology, is one of the hot topics that attract the most attention in today's world. A network structure of the 3GPP basically includes a circuit switched domain and a packet switched domain, and the network structure adopts a structure that is similar to that of a second generation mobile communication system, and includes a universal terrestrial radio access network (UTRAN, Universal Terrestrial Radio Access Network), a GSM/EDGE radio access network (GERAN, GSM/EDGE Radio Access Network), a core network (CN, Core Network), and a user terminal (UE, User Equipment). The GERAN/UTRAN are used to implement all radio-related functions, and the CN processes all voice calls and data connections in a general packet radio service/universal mobile telecommunication service (GPRS/UMTS) system, and is also used to implement functions of switching and routing with an external network. The PS domain includes nodes such as a serving general packet radio service supporting node (SGSN, Serving GPRS Supporting Node), and a gateway general packet radio service supporting node (GGSN, Gateway GPRS Supporting Node). As shown in FIG. 1, the GGSN is mainly responsible for interfacing with an external network, and meanwhile, the GGSN is also responsible for implementing transmission of user plane data. The SGSN implements functions of routing forwarding, mobility management, session management, and user information storage. A home location register (HLR, Home Location Register) is used to store user subscription information.
Long term evolution (LTE, long term evolution)/system architecture evolution (SAE, System architecture evolution) is a next generation mobile network developed by the 3GPP, where the LTE is formed by an evolved universal terrestrial radio access network (E-UTRAN, evolved-UTRAN), and includes an eNodeB network element; the SAE is formed by an MME, a serving gateway (SGW, Serving Gateway), a PDN gateway (PGW, PDN Gateway), and a home subscriber server (HSS, Home Subscriber Server); the eNodeB implements radio-related functions; a mobility management entity (MME, Mobility Management Entity) is a control network element of a packet switched domain, and implements mobility-related functions of certification, mobility management, and session management; and the SGW and the PGW are both gateways, but due to different positions in the network, the SGW and the PGW have different functions, and basic functions are to implement session management, data forwarding, and charging information collection. Because the SGW is connected with the eNodeB, the SGW also has the function of local convergence eNodeB connection.
When a non-3GPP network and a 3GPP network coexist, they may be connected through a PGW. A non-3GPP gateway, for example, an enhanced packet data gateway ePDG or a trusted non-3GPP gateway, accesses the PGW through an S2a or S2b interface, and when the S2a and the S2b support a MIP protocol, the non-3GPP gateway serves as a client of a PMIP, and the PGW serves as a server of the PMIP. When the non-3GPP is handed over to the 3GPP in a PMIP manner, in order to maintain session continuity, when the UE registers in the non-3GPP, a used PGW is registered in the HSS, and when the UE is handed over to the 3GPP, the UE indicates to the network that this is a handover, and the network element MME of the core network obtains the registered PGW from the HSS, and then requests to route to the PGW registered by the non-3GPP, thus completing anchoring of the PGW and maintaining the session continuity. This implementation manner is complex, and multiple network elements including the HSS are involved. A solution for handing over from the 3GPP to the non-3GPP is similar.
When the 3GPP is handed over to the non-3GPP in a DSMIP manner, in order to maintain session continuity, the UE obtains IP address information of the PGW at the 3GPP, and then is handed over to the non-3GPP. First, an IP address of a Home Agent of a current service area is obtained from a broadcast message, and then is compared with a stored IP address. If the IP address of the Home Agent of the current service area is the same as the stored IP address, the PGW is directly contacted according to the address; and if the IP address of the Home Agent of the current service area is different from the stored IP address, an HA of the current service area is selected, but in this case, the session continuity may not be maintained. In this scenario, the terminal needs to support a MIP protocol stack, and the UE directly obtains an address of a packet switched gateway, which easily brings risks to network security.